It is known to add a cerium oxide powder as a compression-set improver and a heat-resistant agent to a silicone rubber composition (see, e.g., Japanese Unexamined Patent Application Publication (hereinafter referred to as “Kokai”) S52-14654 and Kokai S60-173050). However, when an uncured silicone rubber composition that incorporates a cerium oxide powder is exposed to a temperature that exceeds 70° C., then, due to the high reactivity of the cerium oxide powder, the uncured silicone rubber composition either acquires high viscosity, or becomes partially cross-linked. In order to obviate the above problems in the production of the silicone rubber composition, that has to be combined with a cerium oxide powder, the mixture of the composition with the cerium oxide powder is either cooled to a sufficient degree directly in a kneader mixer, or cooled in the outflow from the kneader mixer. Such a solution requires the use of specific mixing equipment and does not allow shortening of the production process. Furthermore, cured products obtained from a conventional silicone rubber composition combined with a cerium oxide powder are not transparent, and therefore have a limited scope of practical application.
Kokai 2000-212444 discloses a silicone rubber composition that incorporates a cerium oxide powder in an aqueous dispersion with pH in a predetermined range. However, in an uncured state, this silicone rubber composition has insufficient thermal stability, and in order to obtain a cured body of the composition with improved compression set and resistance to heat at temperatures that exceed 200° C., the cerium oxide must be added in large quantities.